EMBRYOLOGY 213 



dyads each of which is either wholly paternal or wholly maternal, although 

 of the dyads in a given cell some may be paternal, some maternal. The 

 two cells are of very unequal size, however. One contains nearly all of 

 the protoplasm of the primary oocyte, the other very httle indeed. The 

 disparity between them is much greater than the figure indicates. The 

 larger cell is named the secondary oocyte. The smaller cell is never func- 

 tional, and is called the first polar body; it eventually degenerates. 



Second Maturation Division. — In most animals only the secondary 

 oocyte undergoes further division. Occasionally the first polar body also 

 divides, and to complete the comparison with maturation in the male this 

 occasional division is represented in Fig. 164, but the resulting two 

 polar bodies are not functional. 



The division of the secondary oocyte involves the separation of the 

 dyads into their halves. The division of the cytoplasm is again very 

 unequal, so that one small cell, the second polar body, and one large cell 

 are produced. Were it necessary for the large cell to undergo a change 

 of shape, it would doubtless be called an ootid; but its maturation is 

 finished when the second division is completed, and it is therefore called 

 a mature egg. 



Comparison of Maturation in Male and Female. — Comparison of the 

 maturation of spermatozoa with that of eggs reveals that with respect to 

 the chromosomes the two processes are parallel. The chromosomes unite 

 in pairs and at the same time divide so as to produce tetrads. Two 

 rapidly succeeding divisions divide the tetrads into dyads and the latter 

 into single chromosomes. 



The final cells contain half as many chromosomes as did the germ 

 cells before maturation. These chromosomes may be either paternal, 

 or maternal, or paternal and maternal mixed in any proportion. 



The striking feature in which the maturation processes differ in the 

 two sexes concerns the cytoplasm. In the female ihe divisions are very 

 unequal, so that from each germ cell before maturation there are produced, 

 not four functional cells as in the male, but only one functional cell and 

 two or three degenerate ones. 



The Eggs. — When fully formed the germ cells are of various forms in 

 different animals. The eggs are typically spherical or nearly so. Often, 

 however, one diameter is much greater than the others, forming an ellip- 

 soid. In addition to being elongated, the egg may be curved, as in many 

 insects. Or the widest point may be nearer one end of the egg than the 

 other, as in other insects. The egg is usually polarized, one side being 

 the animal pole, the opposite one the vegetative pole. The effect of this 

 polarity is referred to below in the account of cleavage. 



The size of an egg is closely correlated with the amount of yolk (stored 

 food material) it contains. The eggs of sea-urchins and their allies, of 

 the marine worms, the mammals and others are usually small. Among 



